Thermally and Magnetically Programmable Hydrogel Microactuators

Ionic fuel-powered hydrogel actuators for soft robotics

HYPOTHESIS

Hydrogel actuators powered by chemical fuels are pivotal in autonomous soft robotics. Nevertheless, chemical waste accumulation caused by chemical fuels hampers the development of programmable and reusable hydrogel actuating systems. We propose the concept of ionic fuel-powered soft robotics which are constructed by programmable salt-responsive actuators and use waste-free ionic fuels.

EXPERIMENTS

Herein, soft hydrogel actuators were developed by orchestrating the Janus bilayer hydrogels' capacity for swelling and shrinking. Decomposable and easily removable ionic fuels were applied to power the actuators. Swelling tests were used to evaluate the deformability of the hydrogels. Tensile tests were performed to investigate the modulus of the hydrogels. The bonded interface composed of the interpenetrating polymer chains from both hydrogel layers bilayer was evidenced by the optical microscopy and scanning electron microscopy. The ionic conductivities of solutions were determined by a conductivity meter. Furthermore, a range of biomimetic soft robots with various shapes and asymmetrical structures have been designed and fabricated to execute complex functions.

FINDINGS

The programmable actuators powered by ionic fuel exhibit adjustable bending orientations, amplitudes, and durations, along with consistent cyclic actuations enabled by replenishment of the fuel without noticeable loss in performance. Many life-like programmable soft robotic systems were designed, indicating spatiotemporally controllable functions.

Soft Actuators and Actuation: Design, Synthesis, and Applications

Soft actuators are one of the most promising technological advancements with potential solutions to diverse fields' day-to-day challenges. Soft actuators derived from hydrogel materials possess unique features such as flexibility, responsiveness to stimuli, and intricate deformations, making them ideal for soft robotics, artificial muscles, and biomedical applications. This review provides an overview of material composition and design techniques for hydrogel actuators, exploring 3D printing, photopolymerization, cross-linking, and microfabrication methods for improved actuation. It examines applications of hydrogel actuators in biomedical, soft robotics, bioinspired systems, microfluidics, lab-on-a-chip devices, and environmental, and energy systems. Finally, it discusses challenges, opportunities, advancements, and regulatory aspects related to hydrogel actuators.

Photothermal Conversion of the Oleophilic PVDF/Ti3C2Tx Porous Foam Enables Non-Aqueous Liquid System Applicable Actuator

Various physical and chemical reaction processes occur in non-aqueous liquid systems, particularly in oil phase systems. Therefore, achieving efficient, accurate, controllable, and cost-effective movement and transfer of substances in the oil phase is crucial. Liquid-phase photothermal actuators (LPAs) are commonly used for material transport in liquid-phase systems due to their remote operability and precise control. However, existing LPAs typically rely on materials like hydrogels and flexible polymers, commonly unsuitable for non-aqueous liquids. Herein, a 3D porous poly(vinylidene fluoride) (PVDF)/Ti3C2Tx actuator is developed using a solvent displacement method. It demonstrates directional movement and controlled material transport in non-aqueous liquid systems. When subject to infrared light irradiation (2.0 W cm-2), the actuator achieves motion velocities of 7.3 and 6 mm s-1 vertically and horizontally, respectively. The actuator's controllable motion capability is primarily attributed to the foam's oil-wettable properties, 3D porous oil transport network, and the excellent photothermal conversion performance of Ti3C2Tx, facilitating thermal diffusion and the Marangoni effect. Apart from multidimensional directions, the actuator enables material delivery and obstacle avoidance by transporting and releasing target objects to a predetermined position. Hence, the developed controllable actuator offers a viable solution for effective motion control and material handling in non-aqueous liquid environments.

Preparation of Peptide-Based Magnetogels for Removing Organic Dyes from Water

Water pollution by organic dyes represents a major health and environmental issue. Despite the fact that peptide-based hydrogels are considered to be optimal absorbents for removing such contaminants, hydrogel systems often suffer from a lack of mechanical stability and complex recovery. Recently, we developed an enzymatic approach for the preparation of a new peptide-based magnetogel containing polyacrylic acid-modified γ-Fe2O3 nanoparticles (γ-Fe2O3NPs) that showed the promising ability to remove cationic metal ions from aqueous phases. In the present work, we tested the ability of the magnetogel formulation to remove three model organic dyes: methyl orange, methylene blue, and rhodamine 6G. Three different hydrogel-based systems were studied, including: (1) Fmoc-Phe3 hydrogel; (2) γ-Fe2O3NPs dispersed in the peptide-based gel (Fe2O3NPs@gel); and (3) Fe2O3NPs@gel with the application of a magnetic field. The removal efficiencies of such adsorbents were evaluated using two different experimental set-ups, by placing the hydrogel sample inside cuvettes or, alternatively, by placing them inside syringes. The obtained peptide magnetogel formulation could represent a valuable and environmentally friendly alternative to currently employed adsorbents.

Triple-Responsive Soft Actuator with Plastically Retentive Deformation and Magnetically Programmable Recovery

Multistimuli responsiveness and programmable shape recovery are crucial for soft actuators in soft robotics, electronics, and wearables. However, existing strategies for actuation cannot attain power-free shape retention after removing the external energy supply. Here, a self-assembled density deposition method was developed to fabricate an electrothermal-NIR-magnetic triple-response actuator which was composed of cellulose nanofiber/poly(vinyl alcohol)/liquid metal (CNF/PVA/LM) and magnetic polydimethylsiloxane (MPDMS) layer. Interestingly, the large deformation can be controllably fixed and the temporary configuration will be programmable recovered under a magnetic field due to the thermal-plastic transferring behavior of the CNF/PVA/LM. Rolling robot prepared based on soft actuators exhibits good ability to avoid obstacles. In addition, the object handling and programmable release capabilities of the carrier robots demonstrate that this actuation approach will contribute to a better understanding of how to more rationally utilize various stimuli for application purposes.

Fabrication and Actuation of Magnetic Shape-Memory Materials

Soft actuators are deformable materials that change their dimensions or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response, and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common materials for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing it to a homogeneous magnetic field until the gel point is reached. At this point, the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when it is subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.

Biomedical Applications of Deformable Hydrogel Microrobots

Hydrogel, a material with outstanding biocompatibility and shape deformation ability, has recently become a hot topic for researchers studying innovative functional materials due to the growth of new biomedicine. Due to their stimulus responsiveness to external environments, hydrogels have progressively evolved into "smart" responsive (such as to pH, light, electricity, magnetism, temperature, and humidity) materials in recent years. The physical and chemical properties of hydrogels have been used to construct hydrogel micro-nano robots which have demonstrated significant promise for biomedical applications. The different responsive deformation mechanisms in hydrogels are initially discussed in this study; after which, a number of preparation techniques and a variety of structural designs are introduced. This study also highlights the most recent developments in hydrogel micro-nano robots' biological applications, such as drug delivery, stem cell treatment, and cargo manipulation. On the basis of the hydrogel micro-nano robots' current state of development, current difficulties and potential future growth paths are identified.

Self-Assembling Peptide-Based Magnetogels for the Removal of Heavy Metals from Water

In this study, we present the synthesis of a novel peptide-based magnetogel obtained through the encapsulation of γ-Fe2O3-polyacrylic acid (PAA) nanoparticles (γ-Fe2O3NPs) into a hydrogel matrix, used for enhancing the ability of the hydrogel to remove Cr(III), Co(II), and Ni(II) pollutants from water. Fmoc-Phe (Fluorenylmethoxycarbonyl-Phenylalanine) and diphenylalanine (Phe2) were used as starting reagents for the hydrogelator (Fmoc-Phe3) synthesis via an enzymatic method. The PAA-coated magnetic nanoparticles were synthesized in a separate step, using the co-precipitation method, and encapsulated into the peptide-based hydrogel. The resulting organic/inorganic hybrid system (γ-Fe2O3NPs-peptide) was characterized with different techniques, including FT-IR, Raman, UV-Vis, DLS, ζ-potential, XPS, FESEM-EDS, swelling ability tests, and rheology. Regarding the application in heavy metals removal from aqueous solutions, the behavior of the obtained magnetogel was compared to its precursors and the effect of the magnetic field was assessed. Four different systems were studied for the separation of heavy metal ions from aqueous solutions, including (1) γ-Fe2O3NPs stabilized with PAA, (γ-Fe2O3NPs); (2) Fmoc-Phe3 hydrogel (HG); (3) γ-Fe2O3NPs embedded in peptide magnetogel (γ-Fe2O3NPs@HG); and (4) γ-Fe2O3NPs@HG in the presence of an external magnetic field. To quantify the removal efficiency of these four model systems, the UV-Vis technique was employed as a fast, cheap, and versatile method. The results demonstrate that both Fmoc-Phe3 hydrogel and γ-Fe2O3NPs peptide magnetogel can efficiently remove all the tested pollutants from water. Interestingly, due to the presence of magnetic γ-Fe2O3NPs inside the hydrogel, the removal efficiency can be enhanced by applying an external magnetic field. The proposed magnetogel represents a smart multifunctional nanosystem with improved absorption efficiency and synergic effect upon applying an external magnetic field. These results are promising for potential environmental applications of γ-Fe2O3NPs-peptide magnetogels to the removal of pollutants from aqueous media.